evaluation of the impacts of marine salts and asian dust ... · granitic rocks. this suggests that...

Evaluation of the Impacts of Marine Salts and Asian Duston the Forested Yakushima Island Ecosystem, a WorldNatural Heritage Site in Japan

Takanori Nakano & Yoriko Yokoo &

Masao Okumura & Seo-Ryong Jean &

Kenichi Satake

Received: 1 May 2012 /Accepted: 15 August 2012 /Published online: 12 September 2012# The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract To elucidate the influence of airborne mate-rials on the ecosystem of Japan’s Yakushima Island, wedetermined the elemental compositions and Sr and Ndisotope ratios in streamwater, soils, vegetation, androcks. Streamwater had high Na and Cl contents, lowCa and HCO3 contents, and Na/Cl and Mg/Cl ratiosclose to those of seawater, but it had low pH (5.4 to7.1), a higher Ca/Cl ratio than seawater, and distinct87Sr/86Sr ratios that depended on the bedrock type.

The proportions of rain-derived cations in streamwater,estimated by assuming that Cl was derived from sea saltaerosols, averaged 81 % for Na, 83 % for Mg, 36 % forK, 32 % for Ca, and 33 % for Sr. The Sr value wascomparable to the 28 % estimated by comparing Srisotope ratios between rain and granite bedrock. Thesoils are depleted in Ca, Na, P, and Sr compared withthe parent materials. At Yotsuse in the northwesternside, plants and the soil pool have 87Sr/86Sr ratios similarto that of rainwater with a high sea salt component. Incontrast, the Sr and Nd isotope ratios of soil minerals inthe A and B horizons approach those of silicate mineralsin northern China’s loess soils. The soil Ca and P deple-tion results largely from chemical weathering of plagio-clase and of small amounts of apatite and calcite ingranitic rocks. This suggests that Yakushima’s ecosys-tem is affected by large amounts of acidic precipitationwith a high sea salt component, which leaches Ca and itsproxy (Sr) from bedrock into streams, and by Asiandust-derived apatite, which is an important source of Pin base cation-depleted soils.

Keywords Yakushima . Asian dust . Stream water .

Chemical weathering . Sr isotope . Nd isotope .

Ca depletion

1 Introduction

The atmosphere of the Japanese archipelago is rich inmarine aerosols from the surrounding ocean and has

Water Air Soil Pollut (2012) 223:5575–5597DOI 10.1007/s11270-012-1297-z

T. Nakano (*)Research Institute for Humanity and Nature,457-4 Kamigamo Motoyama, Kita-ku,Kyoto 603-8047, Japane-mail: [email protected]

Y. YokooDepartment of Environmental Systems Science, DoshishaUniversity,1-3 Tataratuya,Kyotanabe, Kyoto 610-0331, Japan

M. OkumuraJapan Oil, Gas and Metals National Corporation,1310 Omiya, Saiwai-ku,Kawasaki 212-8554, Japan

S.-R. JeanDepartment of Geoscience, Chonbuk National University,664-1,567-756 Jeonju, Jeollabukdo, South Korea

K. SatakeFaculty of Geo-environmental Science, Rissho University,1700 echi,Kumagaya, Saitama 360-0194, Japan

been adversely affected by acidic pollutants and dustminerals transported from the Asian continent(Hatakeyama et al. 2004; Shimizu et al. 2004; Inoueet al. 2005; Nakano et al. 2006; Seto et al. 2007;Hartmann et al. 2008). Monitoring studies over morethan 10 years have shown the acid rain impact on soiland aquatic ecosystems in the mountainous area ofJapan (e.g., Kurita and Ueda 2006; Nakahara et al.2010). However, few researchers have evaluated theimpacts of atmospheric deposition of continental-derived materials on Japan’s terrestrial and aquaticecosystems. The effects of rain and aerosols on bio-geochemical cycles are so complex that an integratedapproach that considers entire ecological systems assingle interacting units is required to understand theseeffects. Nutrients and other ions in the soil–vegetationsystem and in terrestrial water are ultimately derivednot only from the atmosphere but also from weather-ing of the soil and the underlying bedrock.Accordingly, identification and quantification ofatmosphere- or bedrock-derived materials in plants,soils, and streamwater are important for assessing thebiogeochemical cycles in terrestrial ecosystems.

Rainwater in Japan has 87Sr/86Sr ratios that clearlydiffer from those of the substrate rocks at depositionalsites, and it contains high quantities of Sr and Ca thatare derived from acid-soluble minerals (mainly calci-um carbonate) that originated in the desert and loessareas of northern China (Nakano and Tanaka 1997;Nakano et al. 2006). Sr is a good proxy for Ca (Milleret al. 1993; Ǻberg 1995; Clow et al. 1997), which isessential for plant growth (as are K, P, and Si), and the87Sr/86Sr ratios of water and vegetation are affected bythe ratios present in a basin’s bedrock (Graustein1988; Faure and Mensing 2005). The 87Sr/86Sr ratioand concentrations of dissolved ions in rainwater showtemporal variation (Nakano and Tanaka 1997; Nakanoet al. 2006), whereas those of a stream’s base flow aretemporally invariant and can therefore be consideredto represent year-round water characteristics (Roseand Fullagar 2005). Accordingly, Sr isotopes havebeen utilized as powerful tracers for determining thesources and flows of Ca within soil–vegetation sys-tems (e.g., Miller et al. 1993; Ǻberg 1995; Blum et al.2002) and aquatic systems (e.g., Clow et al. 1997;Shand et al. 2009). Nd isotopes also have considerablepotential as atmospheric and environmental tracers,since the soils in northern China are reported to have143Nd/144Nd ratios (εNd values) that are distinct from

those of many rocks in Japan (Bory et al. 2003;Nakano et al. 2004). Several Sr and Nd isotope studieshave shown that Asian dust minerals are deposited inthe soils of southwestern Japan (Mizota et al. 1992)and Hawaii (Chadwick et al. 1999; Kurtz et al. 2001),but few studies have used both isotopes as biogeo-chemical tracers in terrestrial systems (Pett-Ridge etal. 2009).

Yakushima Island, in southwestern Japan (Fig. 1),became a world natural heritage site in 1993 in recog-nition of its unique and irreplaceable forested ecosys-tem. This island faces the Asian continent across theEast China Sea, and rainfall and some tree (Pinusamamiana) on the island are intensely affected byaerosols from the surrounding sea and by acidic mate-rials, including gases (SOx and NOx) and aerosols,transported from China (Satake et al. 1998; Nakano etal. 2000; Nagafuchi et al. 2001; Kume et al. 2010).The annual average pH of rain on Yakushima is 4.7, avalue equivalent to that on the main islands of Japan(Tamaki et al. 1991; Japan Environmental SanitationCenter 2002). However, the mean annual precipitationon Yakushima ranges from 2,500 to 4,700 mm at loweraltitudes along the coast, and it exceeds 8,600 mm inmountainous areas (Eguchi 1984). These amounts arethree to five times the precipitation on the mainJapanese islands, indicating that Yakushima is receivingproportionally higher total inputs of acidic materials inprecipitation. Further, the geology of Yakushima iswidely composed of granite, which is known to havesmall acid neutralization capacity. Nevertheless, theimpact of rain and dust minerals from the Asian conti-nent on the island’s plants, soils, and streamwater isunclear. This study was undertaken to elucidate thegeochemical and Sr and Nd isotopic characteristicsof Yakushima’s aquatic, soil, and vegetation systemsand their responses to these atmospheric inputs.

2 Study Site and Methodology

2.1 Geography and Geology

Yakushima Island is located 70 km south of Kyushu(30° N, 130° E), Japan’s third largest island. Thissmall island, 132 km in circumference and 503 km2

in area, consists of steep mountains covered by densenatural forests with many cliffs and with many water-falls owing to the large amounts of precipitation. Mt.

5576 Water Air Soil Pollut (2012) 223:5575–5597

III

IV

V

0 5 kmKumage Group

(sandstone, shale)

0 500 km

Pacific Ocean

Sea of Japan

Yakushima

Granite

Yotsuse

I II

3

63

1414

12

9

15

7

64

13

11

52

8

181816

17 19

818282

5051

36

7543

28

24

22

2038

27

2525

21

34

32

31

2323

26

29

30

33

35

76

5253

59

6060

6162

74

6465

6666

69

6867

70

4545

495757

56

48

464742

403939 41

44

7273

80

7877

79848383

85Ambou R.

Miyanoura R.

Kuromi R.

Granite watershedKumage G. watershedTwo rocks watershed

Taino R.

Sampling site of rain

Onoaida

Shiratani-Unsuikyo

Shin-Takatsuka Mt. Tachudake

Ambou

Issou

Fig. 1 Upper map locationof Yakushima Island and thestudy sites: I, Yakushima Is-land; II, Tanegashima; III,northern Kagoshima; IV,Naegi; and V, Tsukuba. Bot-tom map sampling siteslocations and geologicalbackground of Yakushima.Red circles, black triangles,and empty squares representstreamwater sampling pointsin areas with bedrock domi-nated by granite, bedrockdominated by sedimentaryrocks of the Kumage group,and mixtures of the twotypes of rocks, respectively.The large empty squareindicates the Yotsuse samplesite discussed in the text.Three large filled squaresindicate the locations of therainwater monitoring andsampling by Tamaki et al.(1991), Satake et al. (1998),and Nakano et al. (2000).Sampling locations in areaswith granitic bedrock are 23,39, 45, 57, 66, and 83, andthose in areas with Kumagesedimentary rocks are 14,60, and 82. The sites whereboth streamwater and bed-rock were collected areshown in boldface. Dia-monds indicate the samplingsites of soil in Nakano et al.(2001c)

Water Air Soil Pollut (2012) 223:5575–5597 5577

Miyanoura, the highest point on the island, at 1,935 mabove sea level (a.s.l.), is also the highest peak in theKyushu region. The annual mean temperature isaround 20 °C at the coast; this corresponds to themargin between the subtropical and warm temperatezones (Tagawa 1994); however, the temperaturedecreases with increasing elevation, and areas above1000 m a.s.l. receive snow in winter. Accordingly,there are distinct altitudinal zones of vegetation.About 14,000 residents live in small areas ofYakushima, mostly along the coast at elevations lessthan 100 m a.s.l.

The island is composed mainly of Miocene granitesof the ilmenite series that contain orthoclase mega-crysts with maximum lengths of 14 cm, as well asplagioclase, quartz, and biotite, with small amounts ofchlorite, apatite, zircon, tourmaline, muscovite, andilmenite (Sato and Nagashima 1979). Anma et al.(1998) classified Yakushima’s granite into four typeson the basis of its occurrence, texture, and petrochem-istry: the Yakushima main granite, the core granodio-rite, the core cordierite granodiorite, and the corecordierite granite. The Yakushima main granite occu-pies 90 % of the total area of the Yakushima pluton,whereas the other granites are locally distributed. TheYakushima granite body is an intrusion within theKumage Group, which originated in the Paleogeneage and is composed mainly of sandstone and shaledistributed around the periphery of the island. Thesesedimentary materials are sometimes overlain uncon-formably by terrace deposits, talus deposits, andQuaternary alluvium, mainly along the eastern andsouthern coasts. A pyroclastic flow deposit calledAkahoya covers these rocks in some areas.

2.2 Samples

There are three sites (Issou, Tachudake, and Ambou inFig. 1) for monitoring the precipitation chemistry inYakushima. Detailed compositional data are availablefor the Issou site, where rainwater was collected with abulk sampler at intervals of 1 or 2 weeks from 1994 to1996. This site is about 250 m above sea level and 5 mfrom the ground, on top of a building, and trees, asviewed from the sampler, cover less than 30° of thesky (i.e., there is little or no interference from trees).

From 1996 to 1997 we sampled streamwater at 79locations chosen on the basis of their basin geologyduring the baseflow period from summer to autumn

(Fig. 1). These samples were divided into threegroups: those in granite-dominated watersheds, thosein watersheds dominated by the Kumage sedimentaryrock, and those in watersheds that include both typesof rock. For comparison of streamwater quality inrelation with the watershed geology, we sampledstreamwater from several areas with a range of geo-logical conditions and with negligible upstream hu-man activity on Tanegashima Island, which is close toYakushima and composed primarily of sedimentaryrock; in the northern part of Kagoshima Island, whichis composed primarily of granitic rock, sedimentaryrock, and volcanic rock (mostly andesitic); in theNaegi area of Chubu district, which is composedmostly of granite; and in the Tsukuba area of Ibarakiprefecture, which is composed mostly of granite andgabbro (Fig. 1). At each site, the water samples werefiltered through disposable cellulose acetate filterswith a pore size of 0.2 μm; pH and alkalinity weremeasured immediately after sampling.

We also collected eight granite samples at six loca-tions and four samples of the Kumage sedimentaryrocks at four locations (Fig. 1). Soil is well developedon the hills and gentle slopes. At the Yotsuse site in thenorthwestern part of Yakushima, facing the Asiancontinent (Fig. 1), we collected samples of three plantspecies and soil samples at seven depths. This site islocated at the top of a small hill (200 m a.s.l.), wherethe granitic bedrock is deeply weathered to producehorizons in the soil profile; the thicknesses of the Ahorizon and the B horizon were 30 and 170 cm, re-spectively, whereas the C horizon reached a depth ofmore than 500 cm.

2.3 Analysis

We dried about 40 g of soil from each horizon over-night at 105 °C in an oven. The dried samples werethen reacted with 10 % v/v hydrogen peroxide (H2O2)solution in a tall beaker at 70 °C to separate theorganic fraction. The solution was then centrifuged(Kokusan Enshinki, H-103N Series) at 2,400 rpm for30 min. The supernatant was used for the Sr isotopeanalysis. The residual fraction was washed with ultra-pure water; after centrifugation for 30 min, this super-natant was then discarded. We collected residual soilsafter repeating this rinse procedure three times. Threesoil fractions (<2, 2 to 20, and >20 μm) were separatedfrom about 10 g of the residual soil by means of


Stokes’ law gravity sedimentation in deionized water.They were then concentrated by centrifugation. Bulksoils and these fractions were digested with a solutionof HF, HClO4, and HNO3. We also extracted soilsamples of about 0.5 g with 1 N acetic acid (HOAc)solution to remove the exchangeable fraction. Theremaining solution was used for the Sr isotope analy-sis. Rock samples were pulverized in a tungsten car-bide vessel with a HERZOG HSM-F36 disk mill(HERZOG Automation Corp., Osnabrück, Germany)to obtain powdered samples for chemical and Sr iso-tope analysis. All reagents used in this leaching anddissolution procedure were of analytical grade orbetter.

Chemical analyses were performed at the ChemicalAnalysis Center and the Institute of Geoscience,University of Tsukuba. The concentrations of cationsand anions in streamwater were determined by meansof inductively coupled optical emission spectrometry(Jarrell Ash ICAP-757V, Kyoto, Japan) and aYokokawa Analytical Systems (Yokogawa, Japan)IC7000 ion chromatograph, respectively. The chemi-cal compositions of the rocks and soils were deter-mined by means of X-ray fluorescence with a PhillipsPW1404 analyzer. We determined Sr and Nd isotoperatios by using a Finnigan MAT 262RPQ mass spec-trometer at the University of Tsukuba and a ThermoFisher TRITON mass spectrometer at the ResearchInstitute for Humanity and Nature. The mean87Sr/86Sr ratio of nine standard NBS987 samples dur-ing this study was 0.710246 (2σmean, ±0.000022; n09)using the MAT262 RPQ and 0.710278 (2σmean,±0.000012; n05) using the TRITON, and all measure-ments were normalized with respect to the recommen-ded 87Sr/86Sr ratio of 0.710250. The 143Nd/144Nd ratioof the La Jolla standard was 0.511846±0.000011(2σmean, n012).

3 Results and Discussion

3.1 Streamwater System

3.1.1 Geochemical Characteristics of YakushimaStreamwater

Streamwater was classified into three types based onthe geology of the upstream watershed of the samplingpoint. The chemical compositions of dissolved ions in

the streamwater of Yakushima (Table 1) showed a largegeographical variation, but did not differ significantlybetween the samples from watersheds with graniticbedrock and those with Kumage Group bedrock. Themean water quality values for streamwater inYakushima for the two type’s watershed geology andthose from the other study areas are summarized inTable 2. Streamwater from all areas except Yakushimawas neutral to slightly alkaline, but there was a tendencyfor the streamwater in granitic watersheds to be slightlymore acidic than those in watersheds with sedimentaryor volcanic rock (Fig. 2); the average pH (±σmean) valuesfor streamwater in the granitic watershed (III, IV, and Vin Table 2) and in watersheds with sedimentary orvolcanic rock (II, III, and V in Table 2) were 6.87±0.28 and 7.26±0.27, respectively.

This difference is consistent with the composition ofgranite, which is composed mainly of minerals that areresistant to chemical weathering (i.e., quartz and potas-sium feldspar) and that thus have a lower capacity tobuffer acids in the rain. One remarkable feature is thatthe Yakushima streamwater was more acidic than that inthe other basins, with pH ranging from 5.4 to 7.1 (anaverage of 6.5) versus a range of 6.7 to 8.0 at the othersites. Furthermore, the streamwater at the other sites wasgenerally a CaHCO3 or NaHCO3 type, whereas theYakushima streamwater was generally a NaCl type.The average Na and Cl concentrations in theYakushima streamwater were about 7.6 and 4.7 timesthe average Ca and HCO3 concentrations, respectively.

Monthly analysis of the rainwater composition at theIssou site (Satake et al. 1998; Nakano et al. 2000)revealed that the concentrations of the major dissolvedions were high in winter and low in summer, but that theproportions of Na, Mg, and Cl (Fig. 2 in Nakano et al.2000) were roughly constant throughout the year andwere almost identical to those in seawater, indicatingthat these three ions are largely of sea salt origin. Thenon-sea salt (NSS) Ca and K fractions in the Yakushimarainwater were 0.6±0.2 and 0.3±0.2, respectively(Fig. 4 in Nakano et al. 2000). Nakano et al. (2000)suggested from their Sr isotope study that the NSS Ca isderived mainly from plant cover on Yakushima thatdominantly contains Sr with a marine isotopic signature.Table 2 provides the mean pH, electrical conductivity,and concentrations of the main ions in precipitation.

Chloride is assumed to be a conservative tracer forthe input of sea salt aerosols (Berner and Berner 1987),and the ratio of a given cation to the Cl concentration in


Tab

le1

Chemical

compo

sitio

nandSrisotop

icratio

sof

water

from

individu

alstream

sin

accordance

with

thewatershed

geolog

yin

YakushimaIsland

No.

Elevatio

n(m

)EC

(μS/cm)

Tem

p(°C)

pH87Sr/86Sr

F−

Cl−

NO3−

SO42−

HCO3-

Na+

K+

Ca2

+Mg2

+Si

Sr2+

Ba2

+

μeqmolL−1

neqm

olL−1

Granite

watersheds

1a1190

16.4

9.0

6.00

0.70

840

414

11

2526

132

2137

3575

201

82

2a12

2018

.59.2

6.58

0.70

832

313

90

2168

123

956

3893

247

73

3a1100

17.4

9.7

5.86

0.70

863

416

11

2829

120

936

4452

187

114

4a95

020

.011.6

6.55

0.70

831

315

61

2644

151

941

3612

121

998

5a84

016

.611.8

6.19

0.70

852

313

15

2728

120

1028

3272

158

102

6a66

516

.812

.05.50

0.70

874

4114

245

12119

626

3224

151

70

7a55

519

.612

.46.19

0.70

861

413

93

4132

131

940

3747

208

76

22a

190

51.6

13.6

6.82

0.70

830

336

93

6292

347

2389

8118

134

273

23a

270

57.6

13.1

6.37

0.70

833

345

325

7240

389

2210

310

315

838

890

2451

542

.913

.96.82

0.70

818

228

13

53116

325

2085

6423

333

667

2551

028

.910

.96.50

0.70

849

320

76

4544

186

1152

4810

220

555

2654

036

.611.5

6.65

0.70

843

326

210

6060

250

1569

5414

626

363

2754

036

.513

.26.41

–2

309

1661

3528

912

6766

147

263

64

2864

038

.111.7

6.51

0.70

827

325

26

6376

275

1679

5419

228

563

29a

120

42.1

14.0

6.75

0.70

846

330

64

5980

295

1784

6415

529

557

30a

120

68.0

13.7

6.92

0.70

854

351

76

74116

499

2813

410

824

549

170

35a

220

45.4

16.3

7.13

0.70

847

227

139

4964

256

1984

7614

028

380

39a

625

30.6

13.1

6.70

0.70

848

321

43

3968

225

1855

4717

725

1111

40a

670

27.1

11.5

6.49

0.70

853

322

52

3710

421

414

4050

133

226

102

41a

680

26.7

11.1

6.53

0.70

864

220

96

4148

215

1447

4615

423

380

42a

580

25.5

11.0

6.25

0.70

858

3211

546

4019

58

4348

115

210

76

4420

5–

–6.89

–3

253

8012

430

817

104

5926

241

842

4518

5–

–6.87

–3

273

478

104

324

1985

6423

535

658

4620

0–

–6.41

–3

266

6036

257

1158

5812

924

764

4721

5–

–6.83

0.70

828

225

859

8828

319

7056

212

304

71

4840

––

6.23

–3

236

551

4422

111

5653

117

240

73

49a

255

34.8

14.6

6.06

0.70

862

328

67

4932

267

1745

6112

222

183

56a

3030

.915

.36.61

0.70

843

321

65

4620

213

1360

5014

424

966

57a

7054

.214

.46.95

0.70

846

242

110

6914

041

023

111

9323

646

184


Tab

le1

(con

tinued)

No.

Elevatio

n(m

)EC

(μS/cm)

Tem

p(°C)

pH87Sr/86Sr

F−

Cl−

NO3−

SO42−

HCO3-

Na+

K+

Ca2

+Mg2

+Si

Sr2+

Ba2

+

μeqmolL−1

neqm

olL−1

6395

33.4

15.4

6.53

0.70

842

322

65

5844

234

1671

5114

827

664

6495

33.5

14.5

6.70

0.70

840

322

75

5866

232

1671

5114

727

968

6585

42.6

14.3

6.70

0.70

822

328

915

7284

310

2110

569

216

393

74

6610

32.1

15.1

6.54

0.70

842

421

66

5860

225

1471

5113

727

264

6710

42.8

15.6

6.45

0.70

843

328

78

7268

303

1997

6618

935

470

6814

574

.314

.86.37

0.70

842

457

631

115

4453

435

141

124

209

555

112

6913

056

.514

.16.01

0.70

856

447

821

107

2044

031

114

108

144

461

134

7014

083

.014

.56.18

0.70

850

472

621

142

4464

946

172

161

169

683

124

72a

1630

16.2

7.8

5.36

0.70

844

314

61

268

104

922

3939

189

50

73a

1385

15.9

8.6

6.50

0.70

847

313

521

2898

1037

3866

235

50

7715

033

.813

.66.48

0.70

835

424

16

6247

224

1461

5613

730

835

7820

533

.414

.56.30

0.70

830

323

511

5540

216

1557

5513

930

445

7919

040

.313

.46.72

0.70

829

327

36

6740

270

1787

6318

337

934

8013

037

.113

.26.70

0.70

836

325

52

6010

025

517

7259

163

349

44

8370

50.1

14.1

6.80

0.70

876

333

972

108

357

2195

7424

447

945

8413

544

.613

.16.87

0.70

871

332

767

108

334

1981

7022

843

445

8517

556

.912

.96.07

0.70

875

547

517

998

388

1810

910

412

347

587

Average

37.3

12.9

6.47

0.70

846

327

78

5859

268

1773

6315

031

273

Kum

agegrou

pwatersheds(sedim

entary

rock)

1165

45.5

15.8

6.68

0.71

204

429

89

7384

284

1791

8713

537

286

1224

533

.915

.66.64

0.71

236

321

813

5248

220

1451

6413

830

663

1450

52.7

15.6

6.83

0.71

345

335

711

9896

385

3712

387

161

495

71

1540

54.3

15.0

6.51

0.71

231

433

212

109

8034

723

107

8414

247

773

1610

539

.315

.66.52

0.71

023

327

213

5840

262

2150

7413

331

373

1713

038

.014

.46.43

0.71

218

326

010

6040

227

1765

6910

029

261

1815

37.8

15.6

6.58

0.71

202

325

812

5944

241

2264

68115

290

52

1965

44.0

15.5

6.62

0.71

288

332

55

5768

292

3067

8213

735

270

2014

046

.316

.36.72

0.71

234

333

63

5384

306

2469

8914

439

393

31110

45.3

16.3

6.58

0.71

269

226

314

5368

265

2366

7813

537

082

3212

041

.116

.76.75

0.71

246

224

95

4868

245

1861

7214

632

061


Tab

le1

(con

tinued)

No.

Elevatio

n(m

)EC

(μS/cm)

Tem

p(°C)

pH87Sr/86Sr

F−

Cl−

NO3−

SO42−

HCO3-

Na+

K+

Ca2

+Mg2

+Si

Sr2+

Ba2

+

μeqmolL−1

neqm

olL−1

33115

44.7

16.6

6.78

0.71

337

226

010

5192

260

2378

7616

537

052

3416

038

.817

.36.72

0.7117

92

242

239

4821

716

4765

124

244

54

3835

80.4

15.3

6.51

0.71

041

572

77

107

5265

429

8716

912

645

010

1

4340

47.6

15.9

6.87

0.71

254

331

27

46116

319

28110

9319

445

280

7575

39.4

14.1

6.40

0.71

296

327

45

7652

251

2080

7514

340

651

7610

035

.614

.36.44

0.71

070

324

26

6948

229

1355

6413

636

351

Average

45.0

15.6

6.75

0.71

216

330

79

6566

294

2275

8214

036

969

Mixingwatershedswith

granite

andKum

agegrou

psedimentary

rock

827

023

.214

.86.05

0.70

932

416

31

5312

149

1439

4223

178

77

918

037

.617

.16.84

0.7110

83

229

758

116

228

1710

973

102

370

86

1385

43.8

14.9

6.53

0.70

851

432

711

113

6035

426

8882

166

505

66

2120

25.7

12.2

6.53

–4

183

241

3216

310

4444

7518

350

3665

21.5

16.0

6.40

0.70

871

3119

532

1612

110

3232

7612

857

5014

020

.914

.05.97

0.70

853

314

24

3224

138

1136

3593

167

63

5165

21.9

14.3

6.07

0.70

880

314

75

3432

144

1038

3795

174

61

5270

28.0

15.0

6.22

0.70

869

319

55

5732

204

2147

4486

217

84

5319

026

.014

.26.25

0.70

866

418

35

602

179

1044

4481

187

133

5930

33.8

15.6

6.82

0.70

846

324

25

5064

209

1359

5214

224

760

6050

112.7

17.1

7.05

0.70

882

667

814

146

304

855

4819

116

240

072

860

6160

34.4

17.3

6.28

0.70

842

323

38

5036

219

1652

5313

924

210

2

6229

097

.215

.46.50

0.70

933

495

911

137

4881

942

188

182

196

715

102

7450

29.5

13.3

6.04

0.70

871

3211

550

4019

714

5652

120

295

51

8150

38.7

13.7

6.90

0.70

834

427

04

6268

255

1675

6116

636

144

8210

49.3

14.2

6.54

0.70

836

333

71

7292

334

2189

8118

541

850

Average

41.7

14.8

6.53

0.70

864

430

26

6761

299

1974

6814

432

670

Overallaverage

39.7

14.0

6.54

0.70

940

328

68

6161

277

1873

6814

532

672

Sam

plenu

mbers

referto

thelocatio

nsshow

nin

Fig.1

ECcond

uctiv

ityaThe

site

ofgranite

watershed

inno

rthw

estern

side


Tab

le2

Average

ofelectric

cond

uctiv

ity(EC),pH

,andconcentrations

ofdissolvedions

instream

watersfrom

drainage

basins

with

differentbedrocktypesandthoseof

rain

onYakushimaandseaw

ater

Bedrock

inthe

basin

(area)

Sam

ple

numberEC

(μS

cm−1)

pHSi

Cl−

SO42−

NO3−

HCO3−

Ca2

+Mg2

+Na+

K+

Sr2+

Na/

Cl

Mg/

Cl

Ca/

Cl

K/

Cl

Sr/Cl×

103

Ca/

Na

K/

Na

Mg/

Na

Ca/K

μmolL−1

IGranite

(Yakushima)

5537

6.45

150.3

277.0

29.1

8.4

58.0

36.4

31.3

267.9

16.9

0.16

0.97

0.11

0.13

0.06

0.56

0.14

0.06

0.12

2.16

Sedim

entary

rock

(Yakushima)

2445

6.62

139.9

307.8

32.6

8.5

66.4

37.2

41.2

294.5

22.0

0.18

0.96

0.13

0.12

0.07

0.60

0.13

0.07

0.14

1.69

IaGranite

BDC

24.7

5.4

52.2

10.8

0.47

0.21

0.10

2.29

Kum

ageBDC

24.1

12.8

54.7

15.2

0.44

0.28

0.23

1.59

IISedim

entary

rock

(Tanegashima)

15111

7.14

232.9

680.7

107.1

48.1

433.0

178.7

158.0

726.8

52.1

0.62

1.07

0.23

0.26

0.08

0.91

0.25

0.07

0.22

3.43

III

Granite

(N.

Kagoshima)

1450

7.04

309.8

167.6

52.5

11.6

233.3

126.8

46.1

224.9

26.6

0.45

1.34

0.28

0.76

0.16

2.66

0.56

0.12

0.20

4.77

Sedim

entary

rock

(N.

Kagoshima)

2592

7.38

380.6

189.3

145.3

48.4

554.6

267.7

130.9

368.0

78.0

0.99

1.94

0.69

1.41

0.41

5.25

0.73

0.21

0.36

3.43

Volcanic

rock

(N.

Kagoshima)

3593

7.25

628.8

223.7

99.5

42.9

485.7

223.3

96.4

370.8

78.9

0.58

1.66

0.43

1.00

0.35

2.60

0.60

0.21

0.26

2.83

IVGranite

(Naegi)

426

6.88

199.0

41.5

22.2

8.1

130.0

45.2

9.1

129.6

22.8

0.15

3.13

0.22

1.09

0.55

3.58

0.35

0.18

0.07

1.98

VGranite

(Tsukuba)

376

7.39

459.3

215.5

38.6

46.6

383.6

130.7

43.6

428.9

31.5

0.52

1.99

0.20

0.61

0.15

2.44

0.30

0.07

0.10

4.16

Gabbro

(Tsukuba)

4150

7.91

478.9

215.0

46.9

58.5

1109.7

398.2

170.0

417.6

34.3

0.88

1.94

0.79

1.85

0.16

4.09

0.95

0.08

0.41

11.62

Average

ofrain

atYakushimab

–37

4.70

–157.2

27.7

16.3

–6.7

14.7

122.5

3.45

–0.78

0.09

0.04

0.02

–0.05

0.03

0.12

1.94

Seawater

c–

––

–545,952

28,250

–1,967

10,280

53,086

468,465

10,205

90.16

0.86

0.10

0.02

0.02

0.17

0.02

0.02

0.11

1.01

Num

bers

Ito

Vrepresentthelocatio

nsshow

nin

Fig.1

aAverage

concentrationof

catio

nsin

Yakushimastream

water

contribu

tedby

thebedrock,

granite

andKum

agegrou

pbRainwater

data

areafterNakanoet

al.(200

0)cSeawater

values

arethosepresentedby

BernerandBerner(198

7)


streamwater therefore increases as a result of addition ofthe cation to soil water through chemical weathering.The concentrations of the major cations (Na, K, Ca, andMg) in Yakushima streamwater were positively corre-lated with the Cl concentration (Fig. 3). In addition, theNa/Cl and Mg/Cl ratios of the Yakushima streamwaterwere close to those of seawater (Table 2). Although theCa/Cl and K/Cl ratios of the Yakushima streamwaterwere considerably higher than those of seawater(Table 2), the values were still closer to the seawaterratios than to those of streamwater from other areas ofJapan. These results strongly suggest that the acidicprecipitation on Yakushima contains a substantial seasalt component, which in turn controls the chemicalcomposition of dissolved elements in the Yakushimastreamwater.

The sea salt component of the rain generallydecreases with increasing distance from the coast(Berner and Berner 1987). Tamaki et al. (1991) reportedthe average elemental composition of wet precipitationover 2 years at two Yakushima sites with differentaltitudes, Ambou at 40 m a.s.l. and Tachudake at

475 m a.s.l. (Fig. 1). They found that rainfall onYakushima had a lower annual average Cl concentrationat 475 m than at 40 m, but had the same annual averagepH value (4.7). The concentration of Cl in the stream-water of Yakushima, where the watershed is small,tended to decrease with elevation (Fig. 4). At severallow-elevation sites, the Cl content of the streamwaterwas very high (>0.5 molL−1). Because of the highhumidity that results from the heavy rainfall in the studyarea, this high Cl content in streamwater cannot beexplained only by the concentration process that resultsfrom the evaporation of rainwater; instead, it suggeststhe dry deposition of sea spray in areas near the shore.

The positive correlations of cations in Yakushimastreamwater with the Cl concentration indicate that thecation concentrations tend to decrease with elevation.The altitudinal decreases of Cl and cation concentrationsin streamwater are likely to be caused by the increasedcontribution of rainfall at higher elevations, whichincreases the amount of water relative to the amount ofsea salt. The correlation coefficient between pH andelevation for the Yakushima streamwater is −0.42(P<0.05), suggesting that, although the correlation isweak, streamwater at high elevations tends to be moreacidic (Fig. 4). A similar pattern of decreasing stream-water pH with elevation has been observed in base-poorwatersheds at the Hubbard Brook Experimental Forest(New Hampshire, USA) that have been affected byinputs of acidic deposition (Palmer et al. 2005).Accordingly, the altitudinal decrease of Yakushimastreamwater pH value may be partly ascribed to theeffects of the larger amount of acidic rain because themountainous area receives a high input of H+ ions fromthe atmosphere, and these cannot be fully neutralized byweathering of the granitic bedrock.

NSS sulfate (NSS SO4) is an important componentresponsible for the formation of acid rain. Nakano et al.(2001a) and Ebise and Nagafuchi (2002) reported thatstreamwater in the northwestern side of Yakushima is-land contained higher concentration of NSS SO4 thanthat in the southeastern side (Fig. 5), and attributed thisareal variation of streamwater to acidic deposition trans-ported from the northwestern Asian continent. However,the concentration of non-sea salt cations such as NSS Cain streamwaters of the granite watershed did not show ameaningful difference between the northwestern andsoutheastern sides compared to their altitudinaldecreases. This result suggests that the chemical weath-ering of granite is affected by the amount of precipitation

Fig. 2 Frequency distribution of streamwater pH in (uppergraph) watersheds with granitic bedrock and (lower graph)watersheds with bedrock from the Kumage series of clasticsedimentary rocks, with volcanic rocks (mostly andesitic), andwith gabbro. Vertical dashed lines represent neutral pH (7.0)


and other acids such as carbonic and/or organic acidsgenerating in soil–vegetation system rather than theatmosphere-derived NSS SO4.

3.1.2 Rainwater Contribution to Cationsin Streamwater

When there is no direct contribution from humanactivity, the ions dissolved in the streamwater shouldultimately originate from the atmosphere and from thewatershed’s bedrock. In other words, dissolved ions in

streamwater as well as in the soil water can be sepa-rated into an atmosphere-derived component and abedrock-derived component (BDC), which can besubclassified into granite BDC for the BDC from thegranites and Kumage BDC for the BDC from theKumage sedimentary rocks. It is generally assumedthat Cl in streamwater is derived mainly from precip-itation when there is no volcanic gas or evaporites(both of which are enriched in Cl) in the watershed(Berner and Berner 1987; Nakano et al. 2001b; Négrelet al. 2005). On the other hand, bedrock is generally

0.2

0.4

0.5

0.3

0.1

0.2

0.4

0.5

0.3

0.1

0.7

0.6

0.10

0.20

0.25

0.15

0.05

0.6

1.0

1.2

0.8

0.4

0.2

Fig. 3 Concentrations of the four major cations (Na, Mg, K,Ca) as functions of the Cl concentration in streamwater onYakushima and in other areas with different bedrock geologies.Solid lines indicate values based on the ratios in seawater

(Berner and Berner 1987). The two filled squares represent themean compositions in rainwater at two sites [elevations of475 m (high Cl) and 40 m (low Cl)] on Yakushima in a studyby Tamaki et al. (1991)


considered to be the major source of cations in stream-water through chemical weathering. The concentra-tions of dissolved ions in water increase as a resultof evaporation, but their ratios are generally assumednot to change substantially. Accordingly, the relatively

constant ratios of dissolved cations to Cl in Yakushimastreamwater indicate that the concentrations of theindividual cations derived from bedrock throughchemical weathering increase at a relatively constantrate with decreases in the sea salt component derivedthrough the atmosphere.

2000

500

1000

1500

016040 800 120

NSS Ca

molL-1

0

500

1000

1500

2000

0.7080 0.7082 0.7084 0.7086 0.7088 0.70900

500

1000

1500

2000

0.7080 0.7082 0.7084 0.7086 0.7088 0.709087 86SrSr

Sr isotope

2000

500

1000

1500

020050 1000

NorthwestSoutheast

2000

500

1000

1500

020050 1000

NorthwestSoutheast

molL-1

NSS SO

elevation (m)

Fig. 5 Altitudinal change of concentration of NSS SO4 (top),NSS Ca (middle), and 87Sr/86Sr ratios (bottom) of streamwaterin the granite watershed between the northwestern and south-eastern sides

0.708

granitic rocksedimentary rockmixture of the two rocks

2000

0

500

1000

1500

5.0 5.5 6.56.0 7.0 7.5pH

elev

atio

n(m

)

0

500

1000

1500

2000

rain

stream water

elev

atio

n(m

)

2000

0

500

1000

1500

elev

atio

n(m

)

0.707 0.713 0.7140.7120.7110.7100.709

SrSr

8786

Cl (molL )-11.00.4 0.60.2 8.00

c.r.=-0.42

Fig. 4 Concentration of Cl (top), pH (middle), and 87Sr/86Srratios (bottom) in Yakushima streamwater as a function ofelevation. Most streamwater samples from granite watershedsare from small drainage areas, and this can minimize the effectsof the elevation at which the rainwater falls. The concentrationsof Cl in rainwater at two sites on Yakushima (large filledsquares) were taken from Tamaki et al. (1991)


If all Cl in the Yakushima streamwater is derivedfrom a sea salt component dissolved in rainwater and/or sea spray, and if the uptake of elements by vegeta-tion is negligible, then the proportion of these ele-ments in the streamwater that is derived fromrainwater (frain) can be calculated for a dissolved cat-ion (X, representing Na, K, Ca, and Mg) by using thefollowing equation:

frain ¼ X Cl=ð Þrainwater X Cl=ð Þstreamwater

� ð1ÞUsing the annual average of the compositions of

dissolved ions in Yakushima rainwater (Table 2), wecalculated that 32 % of the Ca and 36 % of the K in thestreamwater of watersheds with granitic bedrock werederived from precipitation, whereas the rainwater con-tributed 81 % of the Na and 83 % of the Mg. In otherwords, two thirds of the Ca and K in the streamwateroriginated from weathering of the granites andKumage sedimentary rock. The average order of dom-inance of the cations in streamwater contributed by thegranite BDC was Na (52.2 μmolL−1)>Ca (24.7 μmolL−1)>K (10.8 μmolL−1)>Mg (5.4 μmolL−1). The or-der for the Kumage BDC was Na (54.7 μmolL−1)>Ca(24.1 μmolL−1)>K (15.2 μmolL−1)>Mg (12.8 μmolL−1; Table 2).

3.1.3 Effects of Chemical Weathering of Graniteon the Yakushima Streamwater

As mentioned above, the chemical composition of thestreamwater did not differ significantly between water-sheds with granite bedrock and Kumage sedimentarybedrock (Table 2). This result can be ascribed to thesimilarity of the dissolved ions in the BDC for bothbedrocks. For example, the average molar ratios of Ca,K, and Mg to Na in the streamwater for the graniteBDC (0.47, 0.21, and 0.10) were generally similar tothose in the streamwater for the Kumage BDC (0.44,0.28, and 0.23). On the other hand, Table 3 showscorresponding ratios of 0.32 for Ca/Na, 0.91 for K/Na,and 0.19 for Mg/Na from the granites, versus 0.32,0.83, and 0.62, respectively, for the Kumage sedimen-tary rock, showing that both rocks have similar Ca/Naand K/Na ratios but very different Mg/Na ratios.

This difference can be ascribed to the chemicalweathering process because the cation compositionsdiffered remarkably between the BDC in streamwaterand in the bedrock. One notable feature is that the

molar Ca/K ratios of the granite BDC (2.28) and ofthe Kumage BDC (1.58) were much higher than thoseof the corresponding rocks (0.39 and 0.61, respective-ly). This contrast is attributable to the higher suscep-tibility of Ca minerals than K minerals to chemicalweathering; thus, the bedrock would become progres-sively depleted of Ca at a faster rate than K.

The mineralogical composition of the granite ismore distinct than that of the sandstone and shale inthe Kumage sedimentary rock. The major cations inthe streamwater of the granite BDC can be largelyascribed to the dissolution of potassium feldspar forK and Na, of plagioclase for Ca and Na, and of biotitefor K and Mg. Potassium is the dominant cation in theYakushima granite but is present at a lower level in thegranite BDC. As the modal composition of K feldsparand plagioclase in the granite is around 30 % respec-tively, whereas that of biotite is about 5 % (Anma et al.1998), this result shows that the potassium feldspardoes not supply enough K into water.

In addition to plagioclase, the Ca in granite issubstituted as accessory minerals such as apatite andcarbonates. The average Ca content of apatite in theYakushima granite is estimated to be 1,180 ppm on thebasis of the P content (an average of 547 ppm;Table 3). Although previous studies have not de-scribed the presence of carbonates in the Yakushimagranite, White et al. (2005) showed that most granitein the world contains at least some carbonates, with anaverage modal ratio of 0.25 %. If the Yakushimagranite contains 0.25 % carbonates, as estimated byWhite et al. (2005), the Ca concentration in the carbo-nates would be 0.1 wt%. On the other hand, the meanCa content of the Yakushima granite was 1.43 wt%(Table 3). Accordingly, the total amount of Ca derivedfrom apatite and carbonates in the Yakushima granitewould be less than 15 %. According to Nakano et al.(2001c), the soils on Yakushima are depleted in Cacompared with the original granite parent material,with the depletion reaching more than 90 % in the Chorizon, which is composed primarily of weatheredgranite. This result indicates that the Ca in the graniteBDC can be attributed mainly to weathering of theplagioclase.

Although plagioclase and potassium feldspar aretwo mineral sources of Na in the streamwater, theaverage molar ratio of K to Ca in the granite BDC ofstreamwater (0.44) is very low compared with that inthe granite (2.97), indicating that the major source of


Tab

le3

Elementalcompo

sitio

ns,Srisotop

icratio

s,andε N

dvalues

ofgranite

andKum

agesedimentary

rock

inYakushimaandloessin

China

Sam

plingsite

number

Si

Ti

Al

MMn

Mg

Ca

Na

KP

Rb

Sr

Zr

Nd

Ca/Na

K/Na

Mg/Na

Ca/K

87Sr/86Sr

87Sr/86Sr*

ε Nd

wt%

ppm

Molar

ratio

Granite

Granite

(avg

.)33

.15

0.33

8.08

2.35

0.05

0.52

1.55

2.39

3.20

603

183

154

165

260.37

30.78

80.20

40.47

40.70

826

0.70

758

-4.12

2329

.24

0.28

8.21

2.04

0.04

0.44

0.99

2.22

6.17

598

246

200

103

–0.25

51.63

50.18

80.15

60.70

810

0.70

763

3931

.05

0.31

7.49

2.22

0.05

0.51

1.72

2.68

3.20

546

173

180

131

–0.36

80.70

10.18

00.52

50.70

818

0.70

762

4530

.75

0.32

7.76

2.25

0.04

0.52

1.47

2.53

3.74

506

169

147

115

–0.33

30.86

80.19

30.38

30.70

827

0.70

761

5730

.98

0.24

7.94

1.70

0.04

0.38

1.68

2.71

3.80

515

161

196

127

–0.35

60.82

40.13

40.43

10.70

810

0.70

740

6631

.78

0.25

7.83

1.82

0.05

0.41

1.55

2.71

3.60

476

207

154

129

–0.32

80.77

90.14

10.42

10.70

817

0.70

774

6630

.36

0.31

7.84

2.20

0.05

0.51

1.23

2.64

4.86

580

216

204

113

–0.26

71.08

30.18

10.24

70.70

814

0.70

760

6630

.96

0.31

7.77

2.61

0.06

0.68

1.06

2.26

3.11

576

157

180

89–

0.27

00.81

00.28

70.33

40.70

821

0.70

755

8331

.98

0.31

7.18

2.16

0.05

0.51

1.58

2.59

2.98

519

161

143

125

–0.35

00.67

50.18

40.51

80.70

818

0.70

767

Average

31.14

0.29

7.79

2.15

0.05

0.50

1.43

2.53

3.85

547

186

173

122

260.32

20.90

70.18

80.38

80.70

818

0.70

760

Kum

agesedimentary

rock

1438

.73

0.22

5.00

2.11

0.02

0.77

0.84

1.61

0.74

305

42118

162

–0.30

00.27

00.45

31.113

0.71

504

−7.52

6029

.44

0.34

8.76

3.02

0.04

1.14

0.96

1.34

3.81

393

160

198

170

–0.41

21.66

90.80

30.24

70.71

606

8233

.20

0.25

6.63

2.92

0.05

1.34

0.95

2.19

2.01

349

8525

312

1–

0.24

90.54

00.57

90.46

10.71

548

Average

33.79

0.27

6.79

2.68

0.04

1.08

0.92

1.71

2.19

349

9619

015

1–

0.32

00.82

60.61

20.60

70.71

553

Chinese

loess

Bulkloess

0.32

5.75

2.75

0.06

1.25

4.81

1.27

1.72

818

8723

186

282.18

00.79

90.93

62.72

90.71

572

−10.31

Loess-1

0.39

6.47

3.07

0.04

1.09

0.66

1.40

1.83

591

133

136

117

270.26

90.76

80.73

90.35

10.71

927

−10.79

Loess-2

0.39

5.70

1.02

0.02

0.49

0.62

1.52

1.70

108

109

143

133

190.23

30.65

70.30

20.35

40.71

979

−11.19

The

“Granite(avg

.)”values

inthefirstlinerepresentstheaveragevalues

for14

major

Yakushimagranites(A

nmaetal.1

998).C

hinese

loessvalues

representthe

averageof

sixloess

samples

from

northenChina

from

Yok

ooetal.(20

04).

87Sr/86Sr*

representstheestim

ated

initial

87Sr/86Srvalues

atan

ageof

14Ma.Sam

plingsitenu

mberscorrespo

nded

tothesites

listedin

Table

1andFig.1.

Loess-1

andloess-2arethemineralsafter5%

HOAcleaching

and20

%HClleaching

ofloess,respectiv

ely


Na in the streamwater is Ca-containing plagioclaserather than potassium feldspar. On the other hand,the average molar Ca/Na ratio of granite BDC instreamwater (0.47) is close to that of the bulk granite(0.32) and plagioclase (0.2 to 1.0; Anma et al. 1998).These data suggest that plagioclase is selectivelyweathered and releases Ca and Na into the water(Berner and Berner 1987). The average molar Mg/Naratio in the granite BDC (0.10) is lower than theaverage molar Ca/Na and K/Na ratios (0.32 and0.91). This result is consistent with the modal ratioof biotite in the granite, which is less than 0.05, andlittle Mg moves into secondary minerals (e.g., chlorite,vermiculite) during the chemical weathering of biotite(Nakano et al. 1991).

3.1.4 Fraction of Rain-Derived Sr in the YakushimaStreamwater

Similarly to the other cations, Sr in the streamwater isderived from both rainwater and the watershed’s bed-rock. We used the following equation for the Sr iso-tope ratio to determine the proportions of the Srderived from rainwater and BDC:

87Sr 86Sr��

streamwater¼ fSr�rain

87Sr 86Sr��

rainwater

þ 1� fSr�rainð Þ 87Sr 86Sr��

BDC

ð2Þwhere fSr-rain is the ratio of the Sr in rainwater to that instreamwater.

The 87Sr/86Sr ratio in streamwater in the granitewatersheds ranges from 0.70818 to 0.70876, whereasthat in the Kumage sedimentary rock watershedsranges from 0.71023 to 0.71345 (Fig. 4). This differ-ence corresponds to the difference in the ratios forgranite (0.70810 to 0.70827) and for the Kumagesedimentary rocks (0.71504 to 0.71606), indicatingthat part of the Sr in streamwater is derived fromweathering of the bedrock in the watershed.Nevertheless, there is no altitudinal change in the87Sr/86Sr ratio in Yakushima streamwater in the gran-ite watershed (Fig. 4). This result suggests that thecontribution of Sr from granite to the streamwater isclose to that from the atmosphere. In contrast, Nakanoet al. (2000) have shown that rain is the dominantsource of seawater-derived Sr on Yakushima, with auniform 87Sr/86Sr value of 0.70918 (Faure and

Mensing 2005). However, it is difficult to determinethe 87Sr/86Sr ratios for granite BDC and KumageBDC, as these ratios are a function of factors such asthe degree of chemical weathering, the Sr contents and87Sr/86Sr ratios of the primary minerals, and the Srcontent of the secondary minerals.

The initial 87Sr/86Sr ratio of the Yakushima granitesat a formation age of 14 Ma was calculated to average0.70760 (Table 3). The 87Sr/86Sr ratio of plagioclaseand apatite in the granite was also estimated to bearound 0.70760, since radiogenic Sr released by thedecay of 87Rb is negligible in these minerals, whichgenerally have a low Rb/Sr ratio, and because of therelatively young age of the Yakushima granite. If the87Sr/86Sr ratio of granite BDC in the streamwater isidentical to that of the bulk granite or plagioclase (plusapatite and carbonates), the value of fSr-rain can becalculated as 0.28 or 0.53, respectively. The formervalue is very close to the value of the rainwater pro-portion for Ca (fCa-rain00.32), which was determinedby using Cl (as described in Section 3.1.3), but theplagioclase-based value is higher.

To determine the value of fSr-rain on the basis of themethod using Cl, we estimated the Sr/Cl ratio ofrainwater by using the following equation:

Sr Cl=ð Þrain ¼ Srsea�salt Clrain=ð Þ þ SrNSS Clrain=ð Þ ð3ÞThis equation can be expressed by using the pro-

portion of NSS Sr in the rain (fNSSSr-rain), as follows:

Sr Cl=ð Þrain ¼ Sr Cl=ð Þseawaterþ fNSSSr�rain 1� fNSSSr�rainð Þ=½ � Sr Cl=ð Þseawater

ð4ÞAlthough there is no report on the Sr content of

Yakushima rain, the good correlation between theconcentrations of Sr and Ca in the rain (r200.88) atfive sites in Japan indicates that it is possible to esti-mate the value of fNSS Sr-rain by using the relationshipbetween NSS Sr/Sr and NSS Ca/Ca, which wasreported by Nakano et al. (2006). As the averageNSS Ca/Ca (weight) of the Yakushima rain wasreported to be 0.6 (Nakano et al. 2000), the fNSS Sr-rain

can be estimated to be 0.15 using Fig. 4 of Nakano etal. (2006). Accordingly, the molar ratio of Sr/Cl forYakushima’s rain can be estimated to be 0.00019 byusing Eq. 4 and the concentrations of Sr and Cl inseawater (Berner and Berner 1987). Because the Sr/Cl


value in Yakushima’s streamwater (mean molar ratiofor granite and sedimentary bedrock00.00057;Table 2) does not differ greatly between the graniteand Kumage sedimentary rock watersheds, the rain-water proportion for Sr in the streamwater can becalculated to be 0.33 from Eq. 1. This value is closeto the value of 0.28 obtained from Eq. 2 when the87Sr/86Sr ratio of granite BDC is assumed to be thesame as that of bulk granite rather than the result ofselective leaching of cations from plagioclase andapatite. This result is consistent with that of Flanklynet al. (1991), who reported that the 87Sr/86Sr ratio ofshallow groundwater in Canada was identical to thatof granite within the same watershed. However, the Srisotopic coincidence between granite BDC and bulkgranite is not well explained at present, and possiblereasons for this coincidence must be examined in afuture study.

The major source of Sr in the water of watershedsunderlain by igneous rock is attributed to plagioclasewith accessory apatite and carbonates because of itshigh Sr content and its high susceptibility to chemicalweathering. Nevertheless, the presence of bedrock-derived K (64 %) and Mg (17 %) in the Yakushimastreamwater indicates that potassium feldspar and bi-otite, despite their small Sr contributions, can stillsupply enough Sr into the streamwater to increase its87Sr/86Sr ratio. Thus, the contributions of rainwaterand watershed bedrock to the streamwater can beevaluated by comparing their elemental compositionsand their 87Sr/86Sr ratios.

Figure 5 shows that the 87Sr/86Sr ratio of stream-water in the granite watershed does not differ signifi-cantly between the northwestern and southeasternsides, indicating that the chemical weathering of gran-ite is not always intense in the northwestern sidewhere the deposition of NSS SO4 is high. This resultis consistent with the above-mentioned view that an-thropogenic acid deposition of NSS SO4 from theAsian continent is not a major factor for the altitudinaldecrease of pH in Yakushima streamwater. Residencetime of rainwater would also become short with ele-vation, which leads to the generation of streamwaterwith low pH and dissolved ions. In order to evaluatethe impact of acidic rain on Yakushima’s aquatic sys-tem, it will be necessary to monitor rainfall pH and thequality of streamwater at several sites with differentelevations; currently, no such data are available forYakushima Island.

3.2 Soil–Vegetation System

3.2.1 Contribution of Atmospheric Sr to the Soil–Vegetation System

Similar to the situations with other elements, the Sr interrestrial plants is ultimately derived from the atmo-sphere and the bedrock. In contrast with the ratios ofmany stable isotopes such as Si and Ca, the 87Sr/86Srratio in the TIMS analysis was determined after cor-recting for mass fractionation (Faure and Mensing2005), so the plant Sr should have the same 87Sr/86Srratio as the ratio in the ambient soil solution. Given thedifferent Sr isotope ratios in the two sources, it ispossible to estimate their relative contribution to Srin plants by comparing the variation in 87Sr/86Sr ratiosand using the calculation method proposed byGraustein (1988). This method approximates the val-ues as follows:

87Sr 86Sr��

plant¼ fSr�rain

87Sr 86Sr��

rainwater

þ 1� fSr�rainð Þ 87Sr 86Sr��

BDC

ð5Þ

Graustein (1988) estimated that about 60 to 80 % ofthe Sr in spruce and aspen in the Tesuque watershed ofNew Mexico is derived from precipitation. Likewise,Miller et al. (1993) reported that about 53 % of the Srin the vegetation in a forest ecosystem in theAdirondack Mountains of New York was of atmo-spheric origin.

The 87Sr/86Sr ratios for terrestrial plants onYakushima depend on the underlying bedrock. Theyranged from 0.70861 to 0.70912 (mean00.70892) ongranitic bedrock and from 0.70918 to 0.70935 (mean00.70925) on the Kumage sedimentary bedrock(Nakano et al. 2000). On Yakushima, the 87Sr/86Srratio in rainwater is close to that in seawater (Nakanoet al. 2000). On the other hand, it is reasonable toassume that the chemical composition and Sr isotoperatio of BDC in soil water are similar to those instreamwater; in the granite watersheds, the 87Sr/86Srratio of BDC can be represented by the average ratiofor Yakushima granite (0.70818). If these assumptionsare valid, the proportion of rain-derived Sr in theplants on granite substrates would range from 43 to94 % (mean074 %), indicating a large contribution ofrain-derived Sr in Yakushima’s plants on a granitesubstrate.


The 87Sr/86Sr ratio of Kumage BDC is not easilyevaluated compared with that of the granite BDC,because the Sr contents and 87Sr/86Sr ratios of theconstituent minerals in the Kumage sedimentary rocksare more heterogeneous. However, if the fSr rain instreamwater of the Kumage sedimentary rock water-sheds is the same as that in the granite watersheds(33 %), the 87Sr/86Sr ratio of Kumage BDC can becalculated as 0.71368 by using Eq. 2. By substitutingthis value into Eq. 5, the proportion of rain-derived Sr inthe plants growing on the Kumage sedimentary sub-strate can be calculated to be more than 96 %. Thisvalue increases to 98 % if the Kumage BDC has theaverage 87Sr/86Sr ratio of the Kumage sedimentaryrocks (0.71552). The dominance of rainwater sourcesof Sr in the plants growing in the Kumage sedimentaryrock watersheds can likely be ascribed to the fact thatvegetation in these watersheds is affected strongly bysea salt particles, as these sites are found mainly at lowerelevations near the coast. Thus, plants on Yakushima aremore enriched in rainwater-derived Sr of sea salt originthan streamwater, regardless of the bedrock type.

Table 4 shows the soil chemical composition,87Sr/86Sr ratio of plants, 87Sr/86Sr ratio, and εNd valueof soils at the Yotsuse site, which overlies a granitesubstrate. At this site, the three plant species that wesampled had similar 87Sr/86Sr ratios (0.70911 for theleaves of Japanese cedar, 0.70913 for plants, and0.70915 for the leaves of Japanese red pine). Thesevalues were close to the 87Sr/86Sr ratio of seawater.From Eq. 5, we can calculate that 93 to 97 % of the Srin the Yotsuse plants was derived from sea salt Sr. It isnotable that the 87Sr/86Sr ratio in the plants is close tothat of the exchangeable soil pool at different depths(which ranges from 0.70911 to 0.70920), with theexception of the lower C horizon, where the87Sr/86Sr ratio (0.70884) is close to that of the under-lying granite (Fig. 6). Another notable feature is thatthe 87Sr/86Sr ratio of the bulk soil at the Yotsuse sitedecreases from 0.71552 near the surface to 0.70908with increasing depth. This ratio is generally higherand more variable than the 87Sr/86Sr ratios in theplants and in the exchangeable soil pool. The relation-ship between the Sr isotope ratios in the plants, soil,and exchangeable soil pool is consistent with the viewthat Sr and other nutrients in Yakushima’s plants areaffected strongly by rainwater and are exchanged withthe exchangeable soil pool rather than with the associatedsoil minerals.

3.2.2 Asian Dust Minerals in the Soil

The depth profile of elements in the soil column atYotsuse differed among the elements (Table 4, Fig. 6).Ti and Zr are assumed to be immobile in the soil envi-ronment during the weathering process (Kirkwood andNesbitt 1991). The degree of enrichment and depletionof element x in the soil at this site, termed fx, wascalculated by using the following equation:

fx ¼ Wxsoil W Ti

soil

�� ðWxgr=W

Tigr Þ

.ð6Þ

whereWxsoil andW

Tisoil are the concentrations of x and Ti,

respectively, in the soil and Wxgr and WTi

gr are the

corresponding concentrations in granite.Most elements except Fe were depleted in the soil

column, but the depletion pattern depended on theelement (Fig. 6). Ca, Na, P, and Sr were depleted atall depths in the soil compared with their values in thegranite parent material. The Ca content in the A and Bhorizons declined to less than 10 % of the value in theparent material, and the Na, Sr, and P contents in the Aand B horizons declined to less than 20 % of the levelin the parent material. On the other hand, the degree ofdepletion was less than 50 % for K, Mg, Mn, and Rb,and the magnitude of the depletion decreased withincreasing depth in the C horizon owing to the weakdegree of chemical weathering of minerals containingthese elements at these depths. This pattern shows thatplagioclase and small amounts of apatite and carbonates(White et al. 2005; Hartmann 2009) are more intenselyweathered than potassium feldspar and biotite.

This result is consistent with the results from soilcolumns at two other sites on Yakushima (Shiratani-Unsuikyo and Shin-Takatsuka; Fig. 1). At all sites, thedepletion of Ca, Na, P, and Sr in the soil compared withthe levels in the original granite was large (gener-ally >50 %), whereas the depletion was weak (10 to50 %) for K, Si, Mg, Mn, and Rb. The depletion waslargest for Ca, reaching more than 95 % in the Bhorizon at the Yotsuse and Shiratani-Unsuikyosites. Soils in the C horizon are composed ofprimary and secondary minerals, and the originalgranite texture is well preserved. Accordingly, the depthvariation of elements in the C horizon can be ascribed tochemical weathering, whereas the variations in the Aand B horizons can likely be ascribed to the presence oforganic matter and exotic materials in addition to thesubstrate materials.


Tab

le4

Elementalcompo

sitio

nsandSrandNdisotop

icratio

sof

soilandSrisotop

icratio

sof

plantsat

theYotsuse

site

Soiltype

No.

Depth

from

surface

Si

Ti

Al

Fe

Mn

Mg

Ca

Na

KP

Rb

Sr

Zr

wt%

ppm

Ahorizon

A0

10cm

26.5

0.63

10.3

13.8

0.03

0.58

0.21

0.45

1.56

209

109

57189

A1

20cm

25.8

0.65

11.2

15.0

0.03

0.62

0.11

0.35

1.47

127

103

53187

Bhorizon

A2

40cm

26.5

0.54

11.3

15.1

0.03

0.52

0.11

0.36

1.74

105

110

47170

A3

70cm

27.7

0.41

11.3

15.2

0.03

0.50

0.05

0.36

2.82

105

143

40134

A4

190cm

28.1

0.34

11.6

15.5

0.03

0.46

0.04

0.39

3.14

140

158

42122

Chorizon

A5

500cm

33.5

0.36

7.1

9.5

0.05

0.67

0.42

1.36

4.43

179

208

89150

A6

650cm

30.6

0.30

10.6

14.3

0.04

0.47

0.19

0.70

3.30

249

186

40135

Soiltype

No.

Depth

from

surface

Bulk

>20

μm

2–20

μm

<2μm

1N

NH4Cl

H2O2

87Sr/86Sr

143Nd/

144Nd

ε Nd

87Sr/86Sr

143Nd/

144Nd

ε Nd

87Sr/86Sr

143Nd/

144Nd

ε Nd

87Sr/86Sr

143Nd/

144Nd

ε Nd

87Sr/86Sr

87Sr/86Sr

Ahorizon

A0

10cm

0.715521

0.512279

−7.00

0.711247

0.719919

0.512138

−9.76

0.721750

0.709105

0.709118

A1

20cm

0.512224

−8.08

0.709191

0.709055

Bhorizon

A2

40cm

0.714919

0.512297

−6.65

0.709198

0.709196

A3

70cm

0.512388

−4.88

0.709168

A4

190cm

0.713594

0.512386

−4.91

0.711721

0.512386

−4.92

0.714722

0.512386

−4.92

0.719593

0.709168

0.709223

Chorizon

A5

500cm

0.709079

0.512387

−4.90

A6

650cm

0.512388

−4.88

0.709644

0.512398

−4.68

0.709990

0.512398

−4.68

0.709211

0.512420

−4.26

0.708839

0.709446

Plant

Leavesof

pine

0.709149

Plants

0.709125

Leavesof

Japanese

cedar

0.709105


The 87Sr/86Sr ratio and εNd values of the C horizon atYotsuse were independent of the particle size, andranged from 0.70908 in the upper C horizon to0.70999 and from −4.90 to −4.26, respectively.These εNd values are indistinguishable from those

of the Yakushima main granite (−5.3 to −4.1; Anma etal. 1998), whereas the 87Sr/86Sr ratios in the C horizonare higher than those of the Yakushima main granite(0.70818). This increased 87Sr/86Sr in the C horizonseems attributable to the enrichment by potassium feld-spar, muscovite, and secondary minerals from biotitewith high 87Sr/86Sr ratios. One notable feature is thatthe 87Sr/86Sr ratio in the bulk soil at Yotsuse tends toincrease towards the surface, moving from the B horizonto the A horizon, whereas the εNd value tends to de-crease. This result indicates that the Yotsuse soil con-tains minerals that did not originate in the parentmaterial, with a high 87Sr/86Sr ratio and a low εNd value,and their contribution increases toward the surface of thesoil column.

China’s Central Loess Plateau is a major deposi-tional area for dust minerals emitted from the upwinddesert areas of northern China and southern Mongolia.Loess in this region is composed of minerals producedby wind erosion and by evaporation (mainly calcite butalso anhydrites and halides) that can be dissolved bywater and acetic acid, phosphates and Mg-containingchlorites that can be dissolved by hydrochloric acid(HCl), and silicates and small amounts of oxides thatresist chemical weathering. According to Yokoo et al.(2004), these minerals have distinct 87Sr/86Sr ratios,with values of 0.7111±0.0004 (mean±SD) for evaporiteminerals, 0.7141±0.0004 for phosphates and chlorites,and 0.7195±0.0010 for silicates, whereas silicates haveNd isotope ratios similar to those of the bulk loess.According to Takahashi et al. (2001), the pH(H2O)value of forest surface soil at 1,034 sites in Japan rangedfrom 3.5 to 8.1, with a median of 5.1. Because the firsttwo types of minerals are not stable in Japan’s acidicsoils, it is possible that the acid-resistant silicate miner-als from the Central Loess Plateau are still present in thesoil of Yakushima Island.

Another notable feature is that the 87Sr/86Sr ratiosand εNd values of the Yotsuse soil depended on theparticle size (Table 4, Fig. 6). In the A and B horizons,the 87Sr/86Sr ratio increases from 0.71125 to 0.71172 inthe coarsest particles (>20 μm) to 0.71959–0.72175 inthe finest particles (<2 μm). The particles >20 μm havea relatively constant 87Sr/86Sr ratio, irrespective ofdepth, indicating that they are derived from the granitesubstrate. The 87Sr/86Sr ratio (0.71992–0.72175) andεNd value (−9.76) of the two fine-grained fractions (2–20 and <2 μm) in the upper A horizon are similar tothose of acid-insoluble minerals in the loess of the

200

527.0507.0 0.7200.7150.71087

86SrSr

Chinese loess

granite

600

400

0

Yakushima

Rain water

bulk>20 2-20 <2

leachate

(cm)Depth

200

Chinese loess

granite

600

400

0

Yakushima

bulk

2-41- -4-6-8-10-12

Ndε

(cm)Depth

AlTiSi

Mg

Fe

CaNa

P

Mn

K

SrRb

0 0.5 1 1.50 0.5 1.0 1.5

Zr

Degree of enrichment or depletion of elements

Al

TiSi

Mg

FeCa Na P

Mn

K

Sr

Rb

Zr

A

B

C

200

600

400

0

(cm)Depth

(normalized to Ti)

mmm

>20 2-20 <2 m

mm

Fig. 6 Top: profiles of the degree of enrichment and depletionof the soil elements (relative to a normalized value of 1.0 in thegranite parent material, represented by the dashed line) as afunction of depth in the soil at the Yotsuse site. Middle: profilesof the 87Sr/86Sr ratio and (bottom) of the εNd values in the soil(for three particle size classes) as a function of depth and in thesoil leachate. The C horizon is composed of weathered granite


Central Loess Plateau (average, 0.72063 and −12.5).Because the average diameter of Asian dust particlesfalling on Japan is about 4 μm (Nagoya University1991), and because larger particles are less likely to betransported over long distances, the contribution ofAsian dust would increase with decreasing particle size.The dependence of the Sr and Nd isotope ratios onparticle size and depth in the Yotsuse soil columnstrongly suggests that the exotic minerals in the soiloriginated from silicates present in Asian dust and thatthe proportion of this foreign dust increases in the uppersoil horizons.

We evaluated the granite-derived and dust-derivedminerals in the Yotsuse soil by using the concentra-tions and isotope ratios of Sr and Nd in the Yakushimagranite and in Chinese loess silicates (Fig. 7). Weassumed two different 87Sr/86Sr ratios and εNd valuesfor each of these components on the basis of theobserved variation for the Chinese loess silicates andfor the Yakushima granites and weathered silicates.The Sr and Nd contents of the dust minerals wereassumed to be 150 and 20 ppm, respectively, whereasthose of the granite-derived components were as-sumed to be 30 and 26 ppm, respectively, becausethe Sr content of the weathered granite is significantly

lower than that of the granite parent material. On thebasis of these assumptions, the 87Sr/86Sr ratios and εNdvalues for the bulk soil and soil minerals with differentparticle sizes fell within a region between two linesthat connect the range of values for the two compo-nents. Figure 7 shows that the contribution of Asiandust silicates ranges from 10 to 30 % in the B horizonand 40 % in the A horizon.

The concentration of Ca in silicates from theChinese loess is significantly lower than that in thebulk loess, because Ca in the loess is mainly present incalcium carbonate (Table 3). Because carbonates dis-solve easily in acidic rain but loess silicates, whichhave a low Ca content, are acid insoluble, it is likelythat the Asian dust silicates do not play an importantrole in the biogeochemical Ca cycle of the Yakushimasoil–vegetation system. In contrast, the concentrationsof P in the bulk loess and carbonate-free (acetic acidinsoluble) minerals were 818 and 591 ppm, respec-tively, versus only 108 ppm in the silicate (hydro-chloric acid insoluble) minerals (Table 3). Becauseapatite will dissolve only in an acid solution strongerthan that required to dissolve calcium carbonate (Joneset al. 1998), it does not dissolve substantially at the pHfound in acidic rain. Most phosphorus in the Chineseloess (>80 %) is present as phosphates. Phosphorus inAsian dust plays a dominant role in regulating vege-tation growth on base-poor soils that have undergonechemical weathering (Chadwick et al. 1999; Hartmannet al. 2008). It is noteworthy that there are no phos-phate minerals in the upper Yotsuse soil. Accordingly,as long as phosphates in the Asian dust are moreinsoluble in acidic rain than carbonates during theirtransport through the atmosphere, the phosphates arelikely to be dissolved in acidic soil and become themajor source of P in the Yakushima soil–vegetationsystem.

3.2.3 Impacts of Atmosphere-Derived Materialson Yakushima Island

The unique topography of Yakushima, with jaggedmountains facing the coast, provides favorable condi-tions for the formation of clouds from water vaporevaporated from the surrounding seawater and sea saltparticles carried into the air by strong winds; this watervapor often condenses to form rain as a result ofcooling as it rises along the mountain slopes. Thisprocess seems to be the primary control on the

Fig. 7 Plot of 87Sr/86Sr versus εNd for soil minerals with threeparticle sizes and for bulk soil at the Yotsuse site. Data for theChinese loess are from Nakano et al. (2004) for HCl residualminerals of surface soils from the Southern Gobi and CentralLoess Plateau. Data for the Yakushima granite are those ofAnma et al. (1998)


chemical composition of rainwater and streamwater,which resembles the composition of diluted seawater.It is considered that the water that originates in the seais not acidic, but is instead somewhat alkaline owingto its high content of sea salts. However, rain is pri-marily acidic due to carbonic acid formed by dissolv-ing CO2 in the atmosphere. It becomes more acidic byincorporation of anthropogenic SOx and NOx fromthe Asian continent, resulting in the formation ofYakushima acidic rain with pH values similar to thosemeasured in other areas of Japan. Large amounts ofacidic rain formed in this manner on Yakushima Islandare favorable for the generation of acidic soil due tocarbonic and organic acids formed in the soil–vegeta-tion system. This geochemical process may acceleratechemical weathering of the Yakushima rocks, leadingto leaching of Ca, Na, and Sr from plagioclase and ofCa and P from apatite, and depletion of these elementsin the soil compared with levels in the parentmaterials.

However, the low concentrations of Ca, Sr, andHCO3 and the high concentrations of H+ in stream-water suggest that chemical weathering (mainly ofplagioclase) is not sufficiently fast to compensate forthe overload of H+ from atmospheric and pedogeneticinputs. Because the rainwater on Yakushima has a lowK content, streamwater K is predominantly of bedrockorigin. Despite its high resistance to chemical weath-ering, alkali feldspar is a major component of thebedrock and is thus a main source of K that is leachedinto the streamwater. Because Ca and P are depleted inthe soil owing to selective weathering of Ca-containing minerals, both elements in the water arederived mainly from atmospheric deposition. The rainis low in Ca because of the dominant sea salt compo-nent. Accordingly, when Asian dust minerals interactwith acidic rain before they arrive on Yakushima,calcium carbonate would dissolve into the acidic rain.

Apatite is a trace mineral and is more acid insolublethan carbonates. Because the apatite is enriched in Caand P, it is a promising nutrient source for the island’svegetation. Primary production of terrestrial ecosys-tem is limited by nitrogen availability (Vitousek andHowarth 1991; Schlesinger 1997). Satake et al. (1998)suggested that nitrogen aerosols and gases such asNOx and NH4 from the Asian continent are potentiallylimiting nutrients for mountainous forests inYakushima. The concentration of NOx in the atmo-sphere of China has recently been increasing owing to

the country’s rapid economic growth, which has beenaccompanied by rapidly growing combustion of fossilfuels (Richter et al. 2005). In addition, Asian dustactivity has also been increasing since the 1990s overeastern China, Korea, and Japan (Chun et al. 2001;Kurosaki and Mikami 2003). Asian dust is known toplay an important role in the rainwater chemistry innorthern China (Xu et al. 2009). Accordingly, moni-toring the inputs of anthropogenic materials and thoseof dust minerals from the Asian continent will beindispensable for evaluating their impacts on thesoil–vegetation system and on the aquatic ecosystemsof Yakushima Island. Understanding these impacts isessential if we are to preserve this important worldnatural heritage site.

4 Conclusions

Yakushima’s rainwater contains dissolved cations thatare highly enriched in the sea salt component, as wellas substantial amounts of anthropogenic S and Ncompounds that lower rainwater acidity. However,even though Yakushima’s rain has an average aciditysimilar to that of Japan as a whole, the island receivesthree to four times the total deposition of atmosphericH+ owing to the large amount of precipitation (4,000to 8,000 mm/year). The substantial impact of acidicrainwater and of seawater on Yakushima’s ecosystemscan be seen in the low pH, Ca, and HCO3 levels andthe high sea salt component in the island’s stream-water. It can also be seen in the Sr isotope ratios ofthe soil water and of land plants, which are close to themarine values. Sr and Nd isotope data further suggestthat the depletion of Ca in the exchangeable soil poolcompared with values in the granitic parent materialsis attributable to selective weathering of plagioclaseand apatite and the recent accumulation of water- andacid-insoluble Asian dust silicates, and that these sec-ondary minerals are not a vital part of the exchange ofbase cations with plants. Future monitoring of thegeochemistry of streamwater, particularly regardingCa losses, will be important to support preservationof the natural heritage site of Yakushima Island.

Acknowledgments This study was supported by a GlobalEnvironmental Fund grant (Acid Precipitation) from the JapanEnvironment Agency and by a Grant-in-Aid for Scientific Re-search Projects from the Ministry of Education, Culture, Sports,Science and Technology of Japan to T. N. (18201004).


Open Access This article is distributed under the terms of theCreative Commons Attribution License which permits any use,distribution, and reproduction in any medium, provided theoriginal author(s) and the source are credited.

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